We have demonstrated that the classical complement pathway is essential to mediate antibody opsonization of pneumococci. We have compared antibody against different components of the cell surface and capsule in their ability to cause C3b binding to pneumococci and shown them to be equivalent. We have compared these C3b deposited in different loci for their ability to bind beta-1H, factor B, factor I and the erythrocyte C3b receptor. These studies have demonstrated on a molecular basis the explanation for the observations that 1) pneumococcal cell walls activate the alternative pathway of complement; 2) pneumococcal capsules do not activate the alternative pathway and 3) anticapsular antibody is more opsonic than anti-cell wall. We have also demonstrated that antibody on pneumococci is a significant acceptor site for C3b during classical pathway activation.